Shouldn't be any problem for you, you can even order barrels from folks like MGM and Bullberry that are already set up like you described.  You may need to order a custom forearm or cut one of your wooden ones down a bit to accomodate the 6" barrel.  That's what I did for my 8" barrels, I just cut the forearm about 1" then reshaped and refinished it.  The forearm is now about 1/4" shorter than the barrel.  Looks like a derringer now!
Since you're talking about a 6" barrel, you might want to use a 1-screw forearm, which will only be about 4" long.  If you're using a G2 frame, you can find some of the old original forearms and modify them to fit the G2.  You just use a coping saw or a hacksaw to open up the notch where the forearm clears the front of the frame - the area right in between the "ears" of the forearm.  Takes a couple of minutes, then rub a little tung oil into the modified area to seal it up.